My experience has been diffferent. I do always add vinegar to the koolaid mix rather than just rely on the citric acid in the koolaid mix.
My favorite dyes are Wilton cake decorating colors. You MUST use vinegar with these, they have no other source of acid. But the colors are much more intense, so you aren't opening 6 packets of strawberry watermelon for a good rich red. The violet color is my favorite for purples, it ranges from a soft lilac to a rich royal blue that is perfect for a night sky. Wilton's also has some more subtle colors premixed, making it easy to get different shades.
I also used vinegar with Koolaid and they aren't washing out. Very happy with them, but would like more color choices (plus it is really hard to find lime Koolaid for some reason!).